Plug valves have been known and used for many years for controlling and modulating the flow of fluids. U.S. Pat. No. 1,166,571 to Bard discloses a three port plug valve with a central key or plug (hereinafter "key") that may be rotated to occlude a selected port. Piping is connected to the various ports of the plug valve and by manipulating the key position, a fluid flowing through a first pipe may be selectively passed through the valve such that another of the connected pipes can be made to communicate with the first. The patent to Bard recognizes that in many applications, one port is dedicated to pressurized inlet flow such that it would be undesirable to occlude the inlet port. In this case, the valve directs the inlet flow to one of the other two remaining (outlet) ports depending upon the position of the key. To protect the inlet port from being occluded by the key, Bard utilizes stops internal or external to the valve. The external stop members act upon the handle of the valve which is attached to the shaft upon which the key rotates.
U.S. Pat. No. 4,470,429 to Johnson discloses another three-way valve which is similar to that of Bard, but which incorporates a symmetrical mounting cover and body adapted to permit the cover assembly to be rotated such that a stop member affixed to the cover can be positioned with respect to a selected port to prevent the occlusion of that port by the internal plug or key. The Johnson valve has a stop depending from a key position indicator on the handle that engages the stop on the cover. While the Johnson valve provides the desirable objective of selecting the port which is excluded from closure, in order to select a different inlet port, the valve must be disassembled, namely, by removing the screws affixing the top cover to the body of the valve and rotating the top cover such that the stop on the cover is aligned with the port which is to be protected by the stop. Accordingly, it would be desirable to provide a plug valve with selectable port exclusion that does not require disassembly and reassembly in order to select the excluded port.